Conventionally, in wireless signal processing, processing is performed in which orthogonal demodulation processing is performed on an intermediate frequency signal (IF signal) to generate a baseband signal and orthogonal modulation processing is performed on a baseband signal to generate an intermediate frequency signal (IF signal) (refer to FIG. 1). Conventionally the orthogonal demodulation processing and the orthogonal modulation processing are performed by using an analog circuit. In recent years, as an LSI is speeded up, a case is increasing, in which the orthogonal demodulation processing and the orthogonal modulation processing are performed by using a digital circuit.
As shown in FIG. 2, in the orthogonal demodulation processing, an I-component of the baseband signal is generated by multiplying the intermediate frequency signal by a cosine wave signal, and a Q-component of the baseband signal is generated by multiplying that by a sine wave signal. As shown in FIG. 3, in the orthogonal modulation processing, an I-component of the baseband signal is multiplied by a cosine wave signal, a Q-component thereof is multiplied by a sine wave signal, and the two signals are added to generate the intermediate frequency signal.
In order to perform the orthogonal demodulation processing and the orthogonal modulation processing by the digital circuit, a numerically controlled oscillator (NCO) which simultaneously generates the cosine wave signal and the sine wave signal is required.
The numerically controlled oscillator normally includes a phase generation unit and a waveform conversion unit (refer to FIG. 4). The phase generation unit generates a time varying phase of a sine wave and a cosine wave to be generated by the waveform conversion unit. The waveform conversion unit converts a phase inputted from the phase generation unit into a sine value and a cosine value corresponding thereto and outputs as a sine wave signal (sin (wt)) and a cosine wave signal (cos (wt)). Since a circuit scale of the waveform conversion unit is large in such numerically controlled oscillator, a configuration thereof is important to save the circuit scale.
For conventional mounting of the waveform conversion unit, four types of methods described below are mainly used.
1. Waveform Table System (e.g. Patent Literatures 1, 2, and 3, Etc.)
The method for calculating a sine value and a cosine value by using a waveform table stored in ROM (Read Only Memory). The feature is to enable high-speed processing.
2. Linear Interpolation System (e.g. Patent Literature 1, Etc.)
The method for adding linear interpolation to the waveform table described above. The ROM table can be decreased compared with the waveform table system. FIG. 14 is a diagram illustrating a waveform conversion unit of a conventional numerically controlled oscillator to which the linear interpolation system is applied. As shown in FIG. 14, a conventional waveform conversion unit 1342 includes a cosine table 1301, a sine table 1302, dividers 1303, 1304, multipliers 1305, 1306, and adders 1307, 1308. The cosine table 1301 and the sine table 1302 store parameters to generate a cosine wave signal and a sine wave signal at a predetermined phase interval, respectively, and a signal of a phase interval which is smaller than the predetermined phase interval is calculated by linear interpolation.
3. CORDIC System (e.g. Patent Literature 4, Etc.)
The method for calculating a coordinate by gradually approaching a target angle by using a difference table of an orthogonal coordinate system. The feature is a small error.
4. Maclaurin Expansion System
The method for directly calculating a cosine wave and a sine wave using a high-order formula in which Maclaurin expansion is performed with respect to a trigonometric function.